One thing that I wanted to experiment was to create a correction curve for digital negatives. Note, this is not a tutorial, there are many out there. There’s a straight forward way with ChartThrob if you have photoshop. With Gimp, you have to do it manually. So I created a grey chart from 0 to 100% white. Then inverted to negative and printed it. I make a cyanotype print. Once it is dry, I scanned it and convert it to grey with minimal change (desaturate). With that file, I measured the average grey for some of the squares. Actually it turns out that I didn’t get an even light on the width of the paper. Decided to just used the vertical line with the values 0, 10, 20, …., 100. That makes 11 point for the curve. Then a bit a calculation in excel, and we get the data for our curve. Save it and then reuse on another picture.
Here are some results, the same negative with the curve applied, on two different papers.
The Hahnemühle Sumi E. This is giving a lot of mid tones
My regular Canson bristol like. With this paper we have bit more highlights. Maybe it worth to expose a bit longer.
And here is the scanned chart. Here we can see the problem that the left column his actually more dense than the right column. This is something I need to fix at some point in order to get more values usable. But a 11 point curve is already a good start. I’ll make a curve with only the left column (10 points) and compare.
